Tuning fork gyroscopes are used to sense angular displacement in military and commercial (e.g., automotive) environments. In one design, two silicon proof masses (vibrating elements) are suspended above a silicon or glass substrate (or a glass substrate with a silicon layer thereon) and there is a conductive (e.g., metal) sense plate (electrode) on the substrate under each proof mass. The inner and outer edges of each proof mass include combs or electrode fingers. Between the proof masses and adjacent to the outer edge of each proof mass are drive motors and sensors with complementary combs or electrode fingers interleaved with the combs of the two proof masses for oscillating the proof masses along a drive axis, i.e., in the direction of the plane of the substrate. Torque and sense transducers are disposed on the substrate as are conductive leads which interconnect the sense plates, the drive motors, and the transducers.
When subjected to angular displacement, one proof mass moves up and the other proof mass moves down (sense direction). By applying a differential voltage to the two sense plates below the masses, the differential displacement of the two proof masses can be detected as the differential capacitance between the proof masses or as currents flowing from the suspended members. An output signal can then be provided which is indicative of the angular displacement sensed by the gyroscope. See U.S. Pat. No. 5,492,596 incorporated herein by this reference.
Unfortunately, steady state acceleration or gravitational inputs in a direction perpendicular to the plane of the substrate causes both proof masses to move away from the substrate resulting in a scale factor error since the output signal depends on the extent of the air gap between the proof masses and the substrate.
Moreover, tuning fork gyroscopes exhibit bias errors as a result of the motor comb drives imparting forces on the proof masses along the sensing axis. Lift force is caused by the asymmetry where the bottom of the drive combs faces a ground plane while the top sees free space. Nominally, both the left and right outer motors exert forces in opposite directions so that the tuning fork (antiparallel mode) proof mass motion is excited. The left and right outer motor combs are mirror images, so that the left and right motor drives exert parallel drive axis forces, and because of differential sensing, these lift forces tend to cancel out. However, the motor drive to sense axis coupling causes two types of bias errors. First, because of tolerance induced construction asymmetries, the left and right drive axis forces will not be equal so that a differential proof mass motion ensues. A second error ensues because of the contact potential (also known as work function) between the metal conductors and the silicon parts. The contact potential adds roughly 0.25 V to the voltage potential between both sense plates and the proof masses. Thus, the common mode motion induced by drive to sense force coupling causes a current flow from the proof mass. This current is interpreted as a bias error.
Generally, increasing the voltage applied to the sense plates increases the scale factor of these instruments. However, this voltage increase is limited by an effect known as snap down. The applied voltage causes electrostatic forces, which act as a negative spring. For the conventional single-sided design, the forces pull the proof mass toward the sense plate, further increasing the negative spring. At sufficiently high voltages, the negative electrostatic spring overcomes the mechanical stiffness and the proof mass unstably snaps into the sense plates. A voltage that is a fraction, typically ⅓ to ⅔ of the snap down voltage, excites the sense plates.
In general then, the asymmetrical design of the prior art tuning fork gyroscopes with sense plates disposed only under the proof masses has been used successfully in many applications. For additional applications, however, still higher scale factor, lower bias error, better bias stability, and lower sensitivity to linear acceleration are required.